The processing of partially crystalline thermoplastic 10 resins is made difficult by the tendency of the resin to crystallize. The inherent properties of the plastic web produced, its degree of crystallinity and its morphological structure are all influenced strongly by the cooling process which is used. A high rate of cooling leads to a finely spherolitic structure, which results in relatively high transparency on account of greatly reduced light scattering. The influence of the cooling rate increases with decreasing crystallization rate, and for this reason, cooling rate is of great importance, with polypropylene for instance, as well as with other vinyl, styrenic or polyester resins that happen to be semicrystalline.
In known processes for calendering plastic web extruded from a slot die, the cooling rate is determined by the temperature of the cooled calendering roll, aside from the effect of the temperature of the surroundings. If the cooling rate thus determined is too low, then the finished plastic web loses its transparency Since the cooling rate decreases with increased roll speed, the roll speed must be limited and thus the production rate must be limited.